In electrophotography (sometimes more generally referred to as electrostatography), an image comprising a pattern of electrostatic potential (also referred to as an electrostatic latent image), is formed on a surface of an electrophotographic element and is then developed into a toner image by contacting the latent image with an electrographic developer. If desired, the latent image can be transferred to another surface before development. The toner image is eventually transferred to a receiver, to which it is fused, typically by heat and pressure.
Toners typically contain a binder and other additives, such as colorants. Binders are generally polymeric and are selected so as to provide a balance between various conflicting constraints. For example, the melt viscosity and melt elasticity of a toner incorporating the binder must not be so low as to cause problems in transferring and fusing a toner image to a receiver.
In U.S. Pat. No. 3,938,992 there is disclosed a toner binder which is crosslinked. However, when a toner based on such a binder is used in a developer, there is an increase in the developer electrical resistance as the developer is continuously used over time to make many images. The term "developer electrical resistance" is commonly used in electrophotography to measure the electrical resistance of the developer wherein the developer comprises toner and carrier particles. The developer electrical resistance is an indication of the magnitude of the electrical field across the carrier and the photoconductor surface when a certain development voltage is applied across the toning shell. The conductivity of a developer is also helpful in removing the counter charge present on the carrier surface once the toner has been developed to the photoconductor surface. The theoretical discussion of developer electrical resistance in conductive magnetic brush development is described in pages 168 through 173 of Electrophotography and Development Physics by L. B. Schein, Springer-Verlag, NY.(1985).
The increased electrical resistance leads to a decrease in the electric field between the toning station and the photoconductive element which results in the transfer of less toner to the photoconductive element for a given charge, and therefore, decreased toner density, "line thinning", or the tendency of alpha numeric characters to become less legible in the fused toner images. Further, these binders have a limited wettability to several compounds which are useful toner addenda, such as iron oxide.
In U.S. Pat. No. 4,473,628, Kasuya et al. discloses a toner composition made from a binder resin produced by emulsion polymerization of butadiene, styrene, and divinyl benzene as a dispersion in water with emulsifiers and initiators. That binder resin is a mixture of two resins, each having different characteristics--one having a low weight average molecular weight, the other a high weight average molecular weight.
Kasuya does not teach a cross-inked polymer but instead teaches the use of high levels of a chain transfer agent (t-dodecyl mercaptan) in conjunction with divinyl benzene) in the preparation of the binder resin (See Example 1, col. 4), a practice known in the art to lower the molecular weight of the polymer as well as prevent the formation of an insoluble (gel) fraction. Such a binder would have certain disadvantages as discussed below.
Cross-linking is desired to impart increased fusing latitude and reduced off-set to the fuser roller despite the fact that increased melt viscosities generally cause deterioration of the fusing properties. Also, cross-linking is desired to reduce the gloss of the fused image.
The incorporation of an insoluble (gel) fraction in the toner binder increases its toughness. As a consequence, the toner breakage caused by mixing the toner with the carrier particles inside the developer station is minimized. Toner breakage, if generated, causes the small toner particles to adhere to the carrier surface which leads to scumming of the carrier surface and eventual rise in developer electrical resistance.
One recently disclosed binder for toner compositions is described in U.S. Pat. No. 5,247,034 (Mate). This reference discloses a binder which is prepared by an emulsion preparation technique using amino acid salts such as ammonium lauryl sarcosinate and sodium lauryl sarcosinate. While these binders are useful, they are not cross-linked and cannot be used in many toner applications in high speed machines because they have very limited fusing latitude and high gloss.
Thus, there is a continuing need for further improvements in toner binders.